there is a lot of variables to selecting a bioindicator. if i was to make a wide sweeping statement on one single entity? than i would say
the species in that ecosystem that is the most moderate.
here is something to read on the subject.
What Makes a Good Bioindicator?
Considering the 1.7 million species that currently documented on Earth, how do we chose just one as a bioindicator? The answer is simple: No single species can adequately indicate every type of disturbance or stress in all environments. Depending upon the specific environment, the species present, and local disturbances, appropriate bioindicator species or groups of species need to be selected. Ecologists have established a broad set of criteria that species must exhibit to be considered good bioindicators (see Table 1).